could pounding and chest thumping because the bicycle ride people love to hate and they're gearing up for their 20th birthday party. joe vasquez is here with a big frustrating, or fun, depending on your point of view, birthday bash, for critical mass. >> a couple thousand people have gathered already, perhaps more than that, you can see the crowd at justin herman plaza and as i walk right at people from different countries, a man from italy, one from brazil, a couple from canada, the costumes are interesting, very colorful, if they're wearing them, there are at least five naked men in the crowd. and the atmosphere is very festive as they're celebrating two decades of taking over the streets of san francisco. >> on the last friday of the month, for 20 years, they have mounted their bicycles and claude san fransisco traffic.